The Naaru are vessels of pure Light, just like how Elementals are vessels of pure Fire, Wind, Water, Earth.

Does this mean that the Naaru are good? No, not really. The Draenei and Naaru visited countless planets during their escape from the Burning Legion, countless planets with much life on them. The Naaru could have saved those beings, instead they left them behind for the Burning Legion to slaughter.

Does this mean the Naaru can cure Undeath? According to the recent Scourge Invasion, yes. As the Alliance was given a gift from the Naaru to combat the crates of plagued grain. However, if you did that questline in Icecrown, sometimes the Naaru get lazy and decide to not cure people. But they will make sure they 'taste Eternal paradise'.